An organic electroluminescent (EL) display, being a kind of light emitting device, is a new type of flat panel display and formed from an array of organic light-emitting diode (OLED) devices. In general, an OLED device is comprised of an electrode serving as the anode, an electrode serving as the cathode and several thin organic layers sandwiched between these two electrodes. The organic layers include at least one emission layer containing more than one fluorescent or phosphorescent organic compound to emit light at each emission color. Applying voltage to the OLED device, holes and electrons are injected from the anode and the cathode, respectively, and form excitons in the emission layer. Then these excitons recombine and release their energy as emission of light.
As one of problems involved in the development of an OLED device, there is a need for an improvement of its emission efficiency. The OLED device generally has a structure in which functional layers including a emission layer are stacked one-dimensionally. In this case, the refractive index (approximately 1.7 to 1.9) of the emission layer is larger than that of the air. Therefore, a majority of light emitted from an inside of the emission layer are totally reflected at an interface of a stacked film at which a high refractive index changes to a low refractive index. The totally reflected light becomes guided-wave light propagating in a horizontal direction (in-plane direction) of a substrate, and then is confined inside the OLED device. A ratio (light extraction efficiency) of light extracted outside the OLED device to the light generated inside the emission layer is generally only approximately 20%. Therefore, in order to improve the emission efficiency of the OLED device, it is important to improve the light extraction efficiency. Japanese Patent Application Laid-Open No. H11-283751 proposes a method of arranging a periodic structure (diffraction grating) in an upper part or a lower part (light extraction side or side opposite thereto) of an organic layer in order to prevent total reflection and suppress optical confinement in a device.
However, in the above-mentioned conventional OLED device, for example, an interference effect of light in a direction perpendicular to a substrate, which is generated by a reflection electrode, is impaired due to an arrangement of the periodic structure, unfortunately leading to a decrease in light extraction efficiency depending on the arrangement of the periodic structure.